A. Technical Field
The present invention relates generally to optical networking technology, and more particularly, to a glass package for optical devices, such as an optical filter.
B. Background of the Invention
The development of optical networking technologies has provided large amounts of network capacity on which data may be communicated. This increased capacity has facilitated the development of high bandwidth applications, including multimedia, voice and video applications, which require large amounts of data to be communicated between network clients. Furthermore, other bandwidth-hungry applications are continually being developed to take advantage of the capacity provided by optical networks.
Wavelength division multiplexing technology, including dense wavelength division multiplexing and coarse wavelength multiplexing, is a significant technological advancement that has resulted in an increase of bandwidth in optical networks. A wavelength division multiplexer launches multiple data channels or wavelengths within a single optical fiber.
These channels are subsequently demuliplexed at a receiver and routed within the network. The number of channels within a WDM network is the primary determinant in the amount of bandwidth within an optical network.
WDM technology also allows a service provider to expand optical network bandwidth without building out or otherwise physically expanding the network infrastructure. In particular, wavelength division multiplexing allows network capacity to be increased by transmitting additional wavelengths in the network. The number and specific wavelengths that may be communicated on the network are limited by the network configuration, the type of optical fiber and optical components within the network.
One such optical component is an optical filter that transmits a particular wavelength(s)and reflects other wavelengths. These optical filters may be applied to pump-signal combining, band splitting, coarse wave division multiplexing, dense wave division multiplexing, CATV, and other networking applications and environments.
An optical filter typically includes a wave division multiplex (“WDM”) filter, a coupling or collimating lens on each side of the filter, and single fiber pigtail or dual fiber pigtail coupled to each of the coupling lenses. The components are surrounded by a housing that protects the components and secures the components within their relative positions.
FIG. 1 illustrates an exemplary housing 100 of an optical filter in which these components may be positioned. The housing 100 includes two metallic collimators tubes 110, 130 and a soldering tube 120 that are generally connected by lead-based solder. The first metallic collimator tube 110 and the second metallic collimator tube 130 contain fiber pigtails and coupling lenses. The soldering tube 120 contains a filter chip, filter cylinder and a filter holder.
An incident light passes through a fiber pigtail in the first metallic collimator tube 110 and is focused onto a coating film on the WDM filter, located in the holder cylinder within the soldering tube 120, by the first coupling lens. A single or group of wavelengths pass through the WDM filter and the second coupling lens focuses the filtered wavelength(s) into the transmission port of the other fiber pigtail in the second metallic collimator tube 130. Other wavelengths may be reflected by the WDM filter into a reflection port of the fiber pigtail by the first coupling lens. It is important that the components within the housing 100 are properly aligned.
The metallic housing 100 may affect the performance of the optical filter as it expands and contracts in relation to temperature. In particular, the coefficient of thermal expansion (“CTE”) of the metal and solder of the housing 100 makes it relatively sensitive to temperature. The expansion/contraction of the housing 100, caused by significant changes in temperature, may affect the position and/or shape of components therein, which may reduce the performance of the filter.
The metallic tubes and lead-based solder within the housing 100 may also contain hazardous materials that are prohibited by various standards or directives. For example, Directive 2002/95/EC of the European Parliament and of the Council of 27 Jan. 2003 on the restriction of the use of certain hazardous substances in electrical and electronic equipment (“RoHS”) which prohibits certain material, including lead, from being within the optical filter must be complied with in order to sell certain electronic components in Europe. Other metallic substances are also banned by RoHS or other standards.
The focusing lenses may affect the performance and size requirements of the optical filter. Various types of lenses have been employed as the coupling lenses within optical WDM filters including C-lenses, gradient index (“GRIN”) lenses, and A-lenses. Each of these lenses has certain optical characteristics that are both advantageous and disadvantageous when applied to the optical filter.
FIG. 2 illustrates an exemplary C-lens that may be used within the optical filter. The C-lens 210 receives an incident light beam and focuses the light on a focus plane 240. The C-lens 210 has a focus length 230 that is defined as the distance between a focusing surface 220 of the C-lens 210 and the focus plane 240. The focus length 230 for the C-lens is typically about 1.9 millimeters. A C-lens 210 typically has a relatively low insertion loss and a CTE of less than 6×10−6/C.
In order for the C-lens 210 to operate properly within a WDM filter, the C-lens 210 must be properly located relative to its focus length 230 and the focus plane 240. Accordingly, the relatively large focus length 230 limits the amount of any reduction in size of the WDM filter.
FIG. 3 illustrates an exemplary GRIN lens that may be used within the optical filter. The GRIN lens 310 receives an incident light beam and focuses the light on a focus plane 330. The GRIN lens 310 has a corresponding distance 340 between a focusing surface 320 of the GRIN lens 310 and the focus plane 330. This distance 340 for the GRIN lens is typically less than 0.25 millimeters. A GRIN lens 310 typically has a CTE of less than 10×10−6/C.
The GRIN lens 310 may not be preferred for high power applications because of irreversible property changes of the dopants during its operation. These dopants may be introduced into the GRIN lens 310 as a result of continuous long-term exposure to intense light.
FIG. 4 illustrates an exemplary A-lens that may be used within the optical filter. The A-lens 420 is surrounded by a metal ring 410 and receives an incident light beam which it focuses on a focus plane 450. The A-lens 420 has a focus length 440 that is defined as the distance between a focusing surface 430 of the A-lens 420 and the focus plane 450. The focus length 440 for the A-lens 420 is typically about 1.9 millimeters.
The use of an A-lens within a WDM filter increases the relative manufacturing cost of the device. This increase in manufacturing cost is caused by the physical structure of the A-lens. Additionally, the A-lens has a relatively longer focus length 440 which may negatively affect the size of the filter.
Accordingly it is desirable to provide an apparatus and method that address the limitations of the prior art.